A thermal flow sensor that measures the flow rate of a gas includes a flow rate detector to measure the flow rate and is configured to measure the flow rate of the gas by allowing thermal conduction between the flow rate detector and the gas to be measured. The flow rate measured by the thermal flow sensor is widely used as an important control parameter of various devices. The thermal flow sensor is characterized in that when compared with other types of flow sensors, the flow rate of a gas can be measured with relatively high accuracy, but in recent years, further improvements of measurement accuracy of the gas flow rate are desired. In vehicles mounted with an internal combustion engine, for example, requirements for fuel saving and requirements for exhaust gas purification are high. To meet such requirements, measurements of an intake air flow as a principal parameter of the internal combustion engine with high accuracy are demanded. A thermal flow sensor that measures an intake air flow introduced into an internal combustion engine includes a sub-passage that takes in a portion of the intake air flow and a flow rate detector arranged in the sub-passage and measures the state of a measured gas flowing through the sub-passage by heat being conducted to the measured gas by the flow rate detector to output an electric signal representing the intake air flow introduced into the internal combustion engine. Such a technology is disclosed by, for example, JP-A-2011-252796 (Patent Literature 1).
Also, a technology to weld a housing and a cover by using laser is disclosed by, for example, JP-A-2007-210165 (Patent Literature 2). Also, as an inspection method of a laser welded portion, a method of determining welding in an expanded state of the welded portion by providing an opening on a transparent resin side is disclosed by, for example, JP-A-2009-056722 (Patent Literature 3). Natural materials containing no carbon material originating from coloring are frequently used for laser welding and depending on the material of resin material or the usage environment, discoloring may occur over time and the appearance may also be important depending on the type of product. Thus, in the Munsell color system, if the lightness is V, the chroma is C, and the hue when a color circle is divided into one hundred portions and a hue 10RP is set as “0” or “100” is H, discoloring over time can be blocked by tinting in color satisfying relationships of “V≤0.229H+3.714, V≤−0.8H+24, V≥3” and “C≤−0.075H2+1.936H+1.267, C≥2”, which is disclosed by JP-A-2010-090234 (Patent Literature 4).